Within the framework of the deployment of mobile or domestic wireless networks, the design of antennas is confronted with a particular problem which stems from the various frequencies allotted to these networks. Specifically, as shown by the non-exhaustive list below, the wireless technologies are numerous and the frequencies on which they are utilised are even more so.
TechnologyApplicationFrequency Band (GHz)GSMMobile telephone0.9DCS 1800Mobile telephone1.8UMTSUniversal mobile system1.9-2.0-2.1DECT - PHSDomestic networks1.8BluetoothDomestic networks2.4-2.48Home RFDomestic networks2.4 ISMEurope BRAN/Domestic networks(5.15-5.35)(5.47-5.725)HYPERLAN2US-IEEE 802.11Domestic networks2.4US-IEEE 802.11aDomestic networks(5.15-5.35)(5.725-5.825)
Thus, the last 20 years have seen the installation of various mobile telephone systems carried on frequency bands which depend on both the operator and on the country of utilisation. More recently, one has witnessed the development of wireless domestic networks with, for certain technologies, a still evolving specification and frequency bands which differ from one continent to another.
From the user's point of view, this multitude of bands may constitute an obstacle to the obtaining of their services in so far as it involves the use of different connection devices for each network. This is why the current trend from the manufacturer's standpoint is aimed at reducing the host of devices by making them compatible with several technologies or standards. Thus we have seen the appearance, a few years ago now, of dual-band telephones which provide for connection both to the 900 MHz GSM and to the 1.8 GHz DCS. Moreover, the multiplicity of standards within the realm of wireless domestic networks is leading to a dividing up of frequency bands which are, either very far apart, or adjacent, depending on the standards under consideration.
In the future, the ever greater demand for frequency spectrum related to the explosion in digital bit rates, on the one hand, and to the scarcity of frequencies on the other hand, will give rise to equipment capable of operating in several frequency bands and/or over a broad band of frequencies.
Moreover, it would be beneficial to develop portable equipment which can be used as a mobile telephone when one is outside one's home and as an item of domestic equipment forming part of the domestic network when one returns home, namely cellular network/domestic network compatible equipment.
It would thus appear necessary to develop antennas operating on several frequency bands so as to allow this compatibility and which are moreover fairly compact.
A planar antenna is currently known which consists, as represented in FIG. 1, of an annular slot 1 operating at a given frequency f. This annular slot 1 is fed by a microstrip line 2.
It has become apparent, following simulations and trials, that if the microstrip line/radiating slot transition is made in such a way that the slot lies in a short-circuit plane of the line, that is to say in the zone where the currents are greatest, then the annular slot will exhibit resonances at all the odd multiples of this frequency, in contradistinction to line-fed structures of the <<patch>> type for which the resonances appear every even multiple of the fundamental frequency. This manner of operation justifies the following design rules which are used to make an antenna as represented in FIG. 1.
In this case,λs=2ΠRIm=λm/4Zant.≈300 Ω
with λs and λm the wavelengths in the slot and under the microstrip line and Zant the input impedance of the antenna. Moreover, I'm represents the length of microstrip line required to produce matching at 50 Ω, Ws and Wm being the width of the slot and the width of the microstrip line respectively.
Thus, in the case of an antenna of the type of that of FIG. 1 made on a <<CHUKOH FLO>> substrate εr=2.6−tanδ=0.002−h=0.8 mm−copper th=15 μm with R=7 mm, Ws=0.25 mm, Im=9.26 mm and operating at a fundamental frequency f of 5.8 GHz, frequency operation as represented in FIG. 2 is observed. A resonance is therefore observed at 5.8 GHz (f) followed by a second resonance at around 17 GHz, namely at 3f, the form of the reflection coefficient remaining flat in the 11 GHz region.